Many thanks for those questions, which I'll try to answer on EB's behalf.

1. Descartes arguably begins from truths, knowledge, & premises not only derived from experience but also constituted & justified altogether independently of experience when he appeals to innate ideas; & the same would be true of Leibniz.

2. The representations of space & time aren't concepts, they're forms of intuition, cognized by pure intution, & therefore judgments about them, or by means of them, aren't analytic, they're synthetic: a kind of cognition not (officially) recognized by the rationalists.

Admittedly, K does in a few places use the term "concepts of space & time," which is misleading: strictly speaking they're forms of intuition & cognized by pure intuition, not concepts cognized by the understanding.

Nevertheless, it's also possible for FORM a general concept of space or time by reflection & abstraction, like all concepts; but this presupposes the pure intuitions of space & time.

In turn, cognition about or by means of the pure intuitions of space & time, in order to be meaningful & true, has to be objectively valid, & it also has to shown to be so by a transcendental deduction which establishes that they're necessary conditions of all possible human experience, as per the Transcendental Aesthetic in the first Critique.

I hope that helps!

Best wishes,

Bob Hanna